In a catalytic cracking process, such as a fluid catalytic cracking (FCC), heavy oils are contacted with a hot cracking catalyst. The hot catalyst vaporizes the oil (usually admixed with steam), then the hydrocarbon vapor and catalyst mixture is carried through a transfer line (riser) to the reactor. Cracking of the heavy oil to lighter components occurs in the riser and the reactor. The catalyst separates from the hydrocarbon in the reactor by means of cyclone separators. The catalyst flows from the reactor to the regenerator where air is injected to burn the carbon from the catalyst. The regenerated catalyst is then ready to be contacted with the hydrocarbon feed again.
The cracked hydrocarbon vapors from the reactor pass to a fractionation tower wherein the hydrocarbon mixture is separated into various product fractions. Some catalyst is carried with the hydrocarbon vapors into the fractionator. The catalyst is concentrated in a heavy oil (slurry oil) in the bottom of the fractionator. In one process disclosed in the prior art, some of the slurry oil is recycled back to the reactor to reduce the loss of catalyst from the plant. The slurry oil is highly aromatic and upon cracking produces larger quantities of light gases and deposits more carbon (coke) on the catalyst than fresh feed. Thus, recycling the catalyst containing slurry oil to the reactor feed conserves catalyst but lowers the product value (light gases and coke are lower in value than slurry oil when used as carbon black feedstock). The recycle stream also reduces capacity of the FCC unit since fresh feed could be fed to the unit instead of the recycle. Gas compressor and air blower capacities are two areas which tend to become overloaded thus limiting FCC plant capacity. The cracking of recycled slurry oil aggravates both areas because the additional light gases produced must be transferred by the gas compressor and the additional coke production requires the air blower to provide more air to burn the coke off of the catalyst in the regeneration step.
It is also known from the prior art that slurry oils can be cleaned by passing these oils through an electrofilter, see for instance U.S. Pat. No. 3,928,158.
There is a continuous need in the art of hydrocarbon cracking for further improvement of the process.